1. Field of the Invention
The present invention relates generally to pneumatic circuits and more particularly to devices for controlling pressure in pneumatic circuits.
2. Description of the Prior Art
In the field of pneumatic circuits it is often desirable to change the pressure in the circuit and to maintain it at the desired pressure until a new pressure setting is required. For example, in an industrial robot where a pneumatically driven robot hand mechanically positions and holds workpieces, it may be desired to have successive or adjustable holding operations requiring different holding pressures. By changing the pressure in the pneumatic circuit driving the robot hand, these different holding pressures can be achieved.
In the past, changing the pressure in a pneumatic circuit has been troublesome. Most controls for the circuit pressure are mechanical. These mechanical pressure control devices usually control pressure by constantly venting through a small orifice and then making up pressure through a valve to a high pressure source when the circuit pressure falls below a set level because of the constant loss. This wastes energy necessary to pressurize the gas which is constantly being vented and produces a constantly changing circuit pressure about the desired pressure.
Other systems mechanically or electrically control valves which vent from the circuit or supply pressurized gas to the circuit based upon upper and lower pressure settings. The problem with these systems is that it is easy to improperly set one of the control settings relative to the other so that they are too close. When this occurs, the system oscillates out of control. Also, since either one of the controls can fail, the system can fail with a maximum pressure supply in the circuit or no pressure in the circuit. Usually, it is desired to have a failure in only one of the two manners so that disastrous results are avoided.
Another problem with controlling pneumatic systems, regardless of the method of control, is cost. The most desirable place for a control device is at a point where the pressure regulation is needed. This, however, means that a separate control device must be positioned at every point of pressure operation. Effective control of a pneumatic circuit of this type thus requires a large number of controllers. Unless the cost of the controllers is relatively low, such a system becomes uneconomical. Thus, although the solutions to the problems described might be possible using expensive equipment, such solutions are not usable because of their cost.
One particular cost problem which relates to the problem of system control is response speed to a signalled or desired pressure change. Generally, it is necessary to have a fast response time so that a desired pressure change will achieve its desired results. However, in systems with both venting and pressuring valves, a fast response means a large "dead band" between the operation of the valves is necessary to prevent uncontrolled oscillation of the two valves. Thus, the fast response time prevents fine control. While a variable orifice valve could be used to provide both fine control and fast response, such valves are expensive and the equipment to drive these valves is also expensive. Finally, such valves are too large for many pneumatic circuit operations.